> For the complete documentation index, see [llms.txt](https://book.bsdcn.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://book.bsdcn.org/man/man9/seqc.9.md). # seqc.9 `seqc_consistent` — 无锁读算法 ## 名称 `seqc_consistent`, `seqc_read`, `seqc_write_begin`, `seqc_write_end` ## 概要 ```c #include ``` ```c void seqc_write_begin(seqc_t *seqcp) void seqc_write_end(seqc_t *seqcp) seqc_t seqc_read(seqc_t *seqcp) seqc_t seqc_consistent(const seqc_t *seqcp, seqc_t oldseqc) ``` ## 描述 `seqc` 允许零个或多个读者和零个或一个写者并发访问对象，为读者提供对象的一致快照。读者和写者之间不需要互斥，但读者可能会被写者无限期地饿死。 `seqc_write_begin` 和 `seqc_write_end` 函数用于为写者创建事务，并通知读者对象将被修改。 `seqc_read` 函数返回当前序列号。如果写者已启动事务，此函数将自旋直到事务结束。 `seqc_consistent` 函数将序列号与先前获取的值进行比较。`oldseqc` 变量应包含读事务开始时的序列号。 读者在事务结束时检查序列号是否已更改。如果序列号未更改，则对象未被修改，获取的变量有效。如果序列号已更改，则对象已被修改，应重复获取。如果序列号为奇数，则对象更改正在进行中，读者将等待直到写操作使序列号变为偶数。 ## 实例 以下写者示例更改 `obj` 结构中的 `var1` 和 `var2` 变量： ```c lock_exclusive(&obj->lock); seqc_write_begin(&obj->seqc); obj->var1 = 1; obj->var2 = 2; seqc_write_end(&obj->seqc); unlock_exclusive(&obj->lock); ``` 以下读者示例从 `obj` 结构中读取 `var1` 和 `var2` 变量。如果序列号已更改，则重新开始整个过程。 ```c int var1, var2; seqc_t seqc; for (;;) { seqc = seqc_read(&obj->seqc); var1 = obj->var1; var2 = obj->var2; if (seqc_consistent(&obj->seqc, seqc)) break; } ``` ## 作者 `seqc` 函数由 Mateusz Guzik 实现。本手册页由 Mariusz Zaborski 编写。 ## 注意事项 不保证读者会有进展。当有大量写者时，读者可能会被饿死。可以通过在几次尝试后返回错误来解决此问题。 理论上，如果读取耗时很长，且有许多写者，计数器可能会溢出并绕回到相同的值。在这种情况下，读者将不会注意到对象已被更改。鉴于这需要在单个争用的读者上进行 40 亿次事务写入，这种情况不太可能发生。可以通过扩展接口以允许 64 位计数器来避免此问题。 --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://book.bsdcn.org/man/man9/seqc.9.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.